docs: def file_exists_on_unit(self, sentry_unit, file_name):
          """Check if a file exists on a unit."""
          try:
              sentry_unit.file_stat(file_name)
              return True
          except IOError:
              return False
          except Exception as e:
              msg = 'Error checking file {}: {}'.format(file_name, e)
              amulet.raise_status(amulet.FAIL, msg=msg)